1. Field of the Invention
The present invention relates to a coil insulator, an armature coil insulated by the coil insulator and an electrical rotating machine having the armature coil, in which hexagonal crystal boron nitride particles having graphitization index of 1.8 or higher are integrated in a mica layer forming a mica tape, the particles being oriented along a longitudinal direction of the armature coil or along a laminating direction of the mica tape.
2. Description of the Related Art
An insulator including mica and thermosetting polymer organic resin as a main component is applied to an armature coil of a large-scale or high voltage electrical rotating machine in many cases. However, the organic resin having inferior heat resistance as compared with metal or ceramic is used as a main constituent material of the insulator, and the insulator is heat-deteriorated by rise of coil temperature by Joule loss during operation of the electrical rotating machine. Thus, various cooling systems for an armature coil are employed in the electrical rotating machine, especially in a large-scale power generator to suppress the heat-deterioration.
The cooling systems are roughly categorized into two kinds, i.e., a direct cooling system in which a conductive body has a hollow structure, water or hydrogen is circulated in the hollow structure and a coil is directly cooled, and an indirect cooling system in which Joule heat generated in the coil conductive body is radiated to an iron core through a coil insulator in addition to a cooling of the iron core by hydrogen or air circulated in the iron core.
Recently, an inexpensive indirect cooling system having a simple structure and excellent maintenance performance has become a focus of attention, and the electrical rotating machine has been increased in capacity by this indirect cooling system.
According to the indirect cooling system, however, since Joule heat is radiated through a coil insulator, the radiation is hindered by the insulator having inferior thermal conductivity as compared with metal or ceramic, and cooling ability is often inferior as compared with the direct cooling system. Hence, a technique for improving the thermal conductivity of the insulator is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-110929 as one example.
FIG. 11 is a diagram for explaining a structure of one example of a conventional coil insulator 110. The coil insulator 110 is formed by winding a plurality of times a mica tape around a coil conductor of an armature coil of an electrical rotating machine (not shown). The armature coil is fixed in a coil slot formed in a stator of the electrical rotating machine, for example.
As shown in FIG. 11, the coil insulator 110 is formed by laminating a plurality of (three, in this case) mica layers 112 each having small mica scales or peeled-off mica flakes 111. Thermosetting resins are impregnated in each of the three mica layers 112 and spaces 113 formed among the mica layers 112, and they are integrally formed as a whole to form the conventional coil insulator 110.
The conventional coil insulator 110 shown in FIG. 11 further includes insulative filler particles 114 having particles diameter of 0.1 μm to 15 μm disposed in the spaces 113 among the mica layers 112. The particless 114 have high thermal conductivity. With this, the impregnated resin layer portion has a specific heat conductivity of at least 5 W/mK.
Here, hexagonal crystal boron nitride particles is used as filler having thermal conductivity of 5 W/mK or more. In this case, this particles material is scaly in shape and has anisotropy in the thermal conductivity, and the thermal conductivity along the crystallographic Z-axis is low. Thus, in terms of improvement in thermal conductivity, it is preferable that the Z-axis of the boron nitride crystal is vertical with respect to the laminating direction of the mica layer 112, i.e., the Z-axis is oriented in a width direction passing through the laminated layer with respect to the laminating direction of the mica layer 112.
Since this material is of scaly in shape, it is preferable that the laminating direction of the mica layer 112 and the Z-axis of the crystal are parallel to each other, i.e., the Z-axis is oriented in parallel with respect to the laminating direction of the mica layer 112 or the longitudinal direction of the coil so that the advancing path length of the electric tree causing the deterioration of the voltage endurance is extended along the longitudinal direction of the laminated mica layer 112. Thus, it is difficult to satisfy both the electric characteristics and thermal conductivity.
Jpn. Pat. Appln. KOKAI Publication No. 55-53802 describes that thermal conductivity of a mica sheet is enhanced when boron nitride particles having particles diameter of 30 μm to 100 μm is mixed in the mica sheet to form an epoxy resin laminated layer, as compared with a product in which boron nitride particles is not mixed. However, when the particles having the particles diameter of 1 μm to 50 μm and mica scales having a size of 0.1 to 1.5 mm are used together to form a mica sheet, the particles is oriented in the same direction as the mica scales in the mica sheet. Thus, when the mica sheet is used as the coil insulation, the hexagonal crystal boron nitride particles is oriented in the laminating direction of the mica sheet or the longitudinal direction of the coil. Namely, it is oriented in a direction in which the thermal conductivity of the boron nitride is low. In this case, although the electric characteristics and thermal conductivity are both improved as compared with a non-boron-nitride-mixed product, there is a room for further improving the thermal conductivity.